Investigating the Return Pathways of North Atlantic Deep Water Using ECCO4 Reanalysis Data

The global meridional overturning circulation (MOC) plays a central role in the regulation of global climate and in the sequestration of carbon and thermal energy into the ocean interior; however, the return pathways for the water cycling through the MOC are difficult to observe directly and current theory relies heavily on inverse modeling and simulation. Developing a clear understanding of the upwelling pathway for North Atlantic Deep Water (NADW) in particular is important as its structure influences the response of the MOC with respect to changes in climatic forcing. Here we use Estimating the Circulation and Climate of the Ocean 4 (ECCO4) reanalysis data to analyze the residual overturning circulation, focusing on the path of NADW as it flows southward from the mid-depths of the Atlantic into the Southern Ocean. We investigate the relative importance of three return pathways for NADW: 1) adiabatic upwelling from the Atlantic interior to the surface along sloped isopycnals in the Southern Ocean, 2) diabatic upwelling within the Atlantic interior, and 3) transformation in the Southern Ocean and eventual diabatic upwelling in the Indo-Pacific. Our results show that adiabatic upwelling dominates the pathway of the upper portion of southward flowing NADW while a large portion of the lower NADW sinks upon entry into the Southern Ocean and cycles into Antarctic Bottom Water before upwelling diabatically in the Indo-Pacific. These results are similar to those derived by Lumpkin and Speer using inverse methods which predict that 11 Sv of the 18 Sv of the southward-flowing NADW is recycled into the abyssal cell upon entry into the Southern Ocean.